Lubricating grease, preparation and use thereof, in particular for lubricating contacts involving elastomers

ABSTRACT

In a particular embodiment, the invention concerns a lubricating grease having 70 to 95 percent by weight of a mixture that includes at least a first ester of fatty acid and of sorbitan, and at least a second ester of fatty acid and of polyoxyalkylated sorbitan. The esters are present in quantities by weight, relative to the weight of the mixture, of 10 to 90 percent and 90 to 10 percent respectively. The grease also includes from 5 to 30 percent by weight of at least one thickening agent.

The present invention relates to a lubricating grease, that is to say alubricating composition that is solid to semi-fluid at ambienttemperature and pressure. More precisely, the present invention relatesto a grease that is particularly suitable for the lubrication ofcontacts in which friction occurs between a metal and an elastomer suchas rubber for example.

The present invention also aims to propose preparation processes forsuch a lubricating grease, as well as uses of this grease inapplications in which it proves to perform particularly well.

The use of lubricating greases is widespread, and a very large number ofcompositions have been described in the prior art.

Thus, U.S. Pat. No. 4,298,481 describes a grease containing from 80 to94% by weight of a base oil constituted by a diester (obtained from C5to C16 alcohols and hydrogenated C32 to C52 dimeric acids), from 0.2 to6% by weight of a mixture of additives including, among others, from0.08 to 2% of an anti-rust agent (preferably sorbitan monooleate) andfrom 5 to 20% by weight of a thickening agent based on modified clay(preferably bentonite) grafted by organic groups. This grease has theadvantage of preserving its properties in the case of prolonged use athigh temperatures (above 200° C.), and it is particularly suitable foruse in aircraft or rocket engines.

U.S. Pat. No. 3,647,690 describes a lubricating grease constituted by anatural or synthetic base oil, thickened with a lithium soap, and theanti-rust properties of which are improved by incorporation of 2 to 5%by weight of an anti-corrosion agent constituted by a metal salt ofnaphthenic or aromatic acid combined with a surfactant having a HLB(“hydrophile-lipophile balance”) coefficient of between 1 and 6. Assurfactant, it is particularly recommended to use alkylated andethoxylated phenols, or esters of fatty acids and of sorbitol (inparticular sorbitol monooleate).

The documents described above thus show that it is known to incorporatein greases small quantities of esters of fatty acids and sorbitan thatare to act as anti-corrosion additives.

Moreover, the greases formulations described above have been optimizedwith a view to improving certain standard properties of greasesessentially intended to lubricate contacts between metals. None answersthe specific problems raised by the lubrication of contacts involvingelastomers.

In a very general way, patent ES 8801941 describes a process for theproduction of greases with a high dropping point, by thickening variousstandard fluids to which are added, at a temperature between 5 and 100°C., from 3 to 20% by weight of a silica-based thickener and from 0.3 to6% by weight of a stabilizing agent. The base fluids can be chosen fromnine large families of oils, including, among others, a family of oilsbased on esters of polyols (containing from 1 to 4 hydroxyl groups) andmonoacids (containing from 4 to 20 carbon atoms). The correspondingexample (Example 4) describes the production of a grease from an esterof pentaerythritol, a compound which is particularly aggressivevis-à-vis rubbers. Such a grease cannot be used at all for thelubrication of contacts in which an elastomer is involved.

In a general way, the greases known in the prior art are very littlesuited to the lubrication of friction contacts between a metal and anelastomer. The formulation of a grease suited to this type ofapplication represents a problem which is all the more delicate becausethe grease must display not only standard properties (such as lubricity,mechanical stability, thermal stability), but also supplementaryproperties including in particular a good compatibility with elastomersand even, for certain applications, an absence of toxicity and abiodegradability.

Pursuing their research into the formulation of novel lubricatinggreases, the Applicant has developed a composition which allows theaforementioned problem to be solved. In particular, the Applicant hasdeveloped novel compositions of greases based on a mixture of two typesof esters of sorbitan in which a thickening agent is incorporated, whichhave proved to constitute particularly useful products, possessingexcellent properties as regards lubricity and mechanical and thermalstability.

Thus, the Applicant has developed a lubricating grease, characterized inthat it contains:

from 70 to 95% by weight of a mixture comprising at least a first esterof fatty acid and of sorbitan and at least a second ester of fatty acidand of polyoxyalkylated sorbitan, these esters being present inquantities by weight, relative to the weight of the said mixture, of 10to 90% and 90 to 10%, respectively; and

from 5 to 30% by weight of at least one thickening agent.

The grease according to the present invention has the major advantage ofbeing wholly compatible with elastomers and plastics. It is evenparticularly suitable for the lubrication of contacts involving thistype of materials, for example contacts between metals and elastomers,or between different types of elastomers or plastics.

In addition to this particular property, the grease according to thepresent invention displays all the standard properties customarilyrequired for greases, including in particular excellent lubricatingproperties, a very good mechanical stability, a high dropping point, astable consistency over a large range of temperatures. As a result, thisgrease can be used to lubricate contacts between very differentmaterials, and its use is not limited to the lubrication of contacts inwhich elastomers are involved.

Other advantages of the present invention will appear on readingpreferred embodiments described in more detail below.

The greater part of the grease according to the present invention isconstituted by a mixture of least two types of esters derived fromsorbitan. The term sorbitan, well known to a person skilled in the art,describes a cyclic tetraol which is an anhydride of sorbitol and can beobtained by dehydration of the latter. By tetraol is meant a polyolcontaining 4 hydroxyl groups.

The first ester is obtained by esterification of one or more fatty acidsand sorbitan.

The second ester is obtained either by esterification of one or morefatty acids and one or more polyoxyalkylated derivatives of sorbitan, orby polyoxyalkylation of esters of one or more fatty acids and sorbitan.In general, the polyoxyalkyl group contains from 1 to 50, preferably 1to 30 units. In general, the alkyl unit contains from 1 to 6, preferably2 to 4 carbon atoms. Preferably, the polyoxyalkyl groups present in thesecond ester contain at least one polyethoxy group.

Each of the two types of ester can comprise monoesters or polyesters.Preferably, the said esters are chosen in such a way that the mixture ofthe two types of esters is liquid at 25° C. and atmospheric pressure.

Various fatty acids can feature in the composition of the said esters.Moreover, the fatty acids featuring in the composition of the said firstester can be identical to, or different from, those featuring in thecomposition of the said second ester. These fatty-acids, which can belinear or branched, saturated or unsaturated, advantageously containfrom 6 to 18 carbon atoms, and preferably from 12 to 18carbon atoms. Forexample, and in a non-limiting way, the said fatty acids can be chosenfrom lauric, palmitic, stearic, oleic acids.

Preferably, the said first ester comprises sorbitan monooleate. Equallypreferably, the said second ester comprises polyethoxylated sorbitanmonooleate.

The mixture of the two types of ester contains for example from 20 to80% by weight, advantageously from 30 to 70% by weight, and preferablyfrom 40 to 60% by weight of the said first ester (ester of fatty acidand of sorbitan), and for example from 20 to 80%, preferably from 30 to70% by weight and preferably from 40 to 60% by weight of the said secondester (ester of fatty acid and of polyoxyalkylated sorbitan). Even morepreferably, the quantities by weight, in the mixture, of each of the twotypes of esters are more or less equal.

The grease according to the present invention also contains at least onethickening agent, the function of which is to give it its solid texture.The level of thickening agent depends on the hardness desired for thegrease, and is between 5 and 30% by weight, and preferably between 10and 20% by weight. A person skilled in the art will be fully aware how,within this range, to optimize the level of thickening agent in order toobtain the requisite degree of hardness.

Various thickening agents, organic or inorganic, can be used. By way ofnon-limiting example, the thickening agent can be chosen from lithiumsoaps, calcium soaps, thickeners based on polyurea, silica, clay.

According to a first preferred embodiment, the thickening agent containsat least a clay grafted by organic groups. Preferably, this clay isbentonite. The term bentonite designates, in a manner known per se, anargillaceous material comprising essentially clays of the smectitesgroup (see “Ullmann's Encyclopedia of Industrial Chemistry”, 1986, Vol.A7, Chap. 2,2, p. 116).

This type of clays has the property of swelling in an aqueous medium,and it is customary to modify these clays by grafting on organic groups,in order to obtain materials capable of being dispersed in an organicmedium. The grafting is generally carried out by ion exchange, replacingthe inorganic ions naturally present on the surface of the clay withions of alkyl-ammonium type.

According to a second preferred embodiment, the thickening agentcontains from 70 to 100% by weight of silica. The term silicadesignates, in a manner known per se, silicon oxide. The silica usedadvantageously has a high specific surface, preferably greater than orequal to 150 m²/g. It is particularly advantageous to use a colloidalsilica, such as that which is obtained by flame hydrolysis of siliconchloride (SiCl₄) in gas phase.

Preferably, a silica is used the composition of which is suited to polarmedia. The silica-based thickener can also contain up to 30% of othercomponents, which are preferably other oxides. These preferably comprisealuminium oxide.

It is customary to add to silica, when this is used as a thickener forgreases, an adjuvant the function of which is to increase the thickeningpower of the silica, forming hydrogen bonds between the silanol groupspresent on the surface of the silica particles. The grease according tothe invention advantageously contains such an adjuvant, at a level ofbetween 0.5 and 2% by weight relative to the total weight of the grease.By way of non-limiting example, this adjuvant can be chosen from water,glycerol, ethylene glycerol, glycol, ethylene glycol. Polyethoxylatedstearates or polystearates of sorbitan, adjuvants known for such anapplication, can advantageously perform this function of adjuvant whilestill forming, where appropriate, a part of the mixture of estersconstituting the base of the grease (alone or mixed with other esters ofpolyalkoxylkated fatty acids).

In the two preferred embodiments described above, the grease has theadditional advantage of displaying complete respect for the environment.These two formulations of greases are in fact biodegradable, and aboveall they pose no risk of pollution of the groundwater.

The pollutant character vis-à-vis the groundwater can be quantified bywhat is called the WGK (from the German “Wasser Gefährdigungsklasse”)index, as defined in the German ministerial directive of 18^(th) Apr.1996 (Federal Ministry for Internal Affairs, Federal Ministry for theEnvironment, Protection of Nature and Reactor Safety; Potential WaterPollution Substances Administrative Order, 18^(th) Apr. 1996). The twogrease formulae described above, in which the thickener is based onorganic bentonite or silica, have a WGK index equal to 1, which reflectstheir non-pollutant character.

If necessary, the grease according to the present invention can alsocontain, in addition to the mixture of esters and the thickening agent,a certain number of additives customarily used in lubricatingcompositions. These additives are generally incorporated in a smallquantity, in order to improve some particular properties, according tothe application for which the grease is intended.

For example, the grease can advantageously contain from 0.25 to 1.5% byweight of an antioxidant additive, which can chosen from, among others,the standard antioxidant additives which are phenolic oramino-compounds, metal dithiocarbamates, metal dithiophosphates.

Other types of additives can also be incorporated in the greaseaccording to the invention, in particular anti-corrosion additives,anti-rust additives, metal passivators, friction modifiers, adhesionagents, additives improving the pour point.

The grease according to the present invention has a grade which dependsof course on the envisaged application. The grade of a grease is, in amanner well known to specialists, usually determined by means of a testcalled the “cone penetrability test”, carried out in accordance withstandard ASTM D-217. The grease according to the inventionadvantageously has a cone penetrability of between 265 and 340 1/10^(th)mm.

Moreover, the dropping point of the grease is preferably high enough toallow it not to liquefy and to continue to perform its function when thetemperature increases. For example, the grease preferably resists localincreases in temperature which are sometimes large, and which are due tothe heating phenomena associated with the frictions between thematerials in contact.

Thus, the grease according to the invention advantageously has adropping point which is greater than or equal to 200° C., and which canbe for example of the order of 260° C. In the present description thequoted dropping points are measured in accordance with standard IP-396.

The greases according to the present invention can be prepared invarious ways. An advantageous procedure is to carry out a first stage ofmixing the said first and second esters (ester of fatty acid and ofsorbitan and ester of fatty acid and of polyoxyalkylated sorbitan),followed by a second, thickening, stage comprising the incorporation ofthe thickening agent and the mixture in conditions suitable for bringingabout thickening.

The conditions in which the second, thickening, stage is carried outdepend on the type of thickener used and the required grade.

In particular, when the thickening agent is based on bentonite graftedby organic groups, it is customary to use a swelling agent of polartype, which allows the swelling of the bentonite to be facilitated bydeploying the organic chains grafted on its surface. Various compoundscan be used to this end, for example water, an alcohol, acetone,propylene carbonate. The quantity of swelling agent used is preferablyof the order of approximately 10% relative to the weight of the graftedbentonite.

The thickening stage can thus be carried out in the following manner:

dispersion of the grafted bentonite in the liquid mixture of the twoesters, at a temperature of 30 to 60° C. and accompanied by stirring,

heating to a temperature of 30 to 60° C., and, at this temperature,addition of a swelling agent in a quantity of between 5 and 20% byweight relative to the weight of bentonite,

stirring at a temperature of 30 to 60° C. at a high shear rate, for 1 to4 hours, incorporation of any additives,

final mixing by grinding in a colloidal mill.

On the other hand, when the thickening agent is based on silica, thethickening stage can be carried out in the following manner:

dispersion of the silica in the liquid mixture of the two esters, at atemperature of 30 to 80° C. and accompanied by stirring;

incorporation of any adjuvant in a quantity of from 0.5 to 2% by weightrelative to the weight of the grease;

mixing at a temperature of 30 to 80° C. at a high shear rate for 1 to 4hours, incorporation of any additives and final mixing.

A person skilled in the art will easily be able to determine the optimumshearing and grinding conditions, according to the components and theequipment used, the sought final hardness, etc., on the basis of hisgeneral knowledge.

Thanks to its excellent properties, the grease that is the subject ofthe present invention can be used as a lubricant in extremely variedapplications. It allows the lubrication with a very good effectivenessof the contacts in which very different materials, metal or not, can beinvolved.

Thanks to its compatibility with elastomers, the grease according to theinvention is particularly suitable for the lubrication of contactsinvolving one or more materials based on elastomers, synthetic ornatural (for example rubber), even plastics.

An application example in which the grease according to the inventionperforms particularly well concerns the lubrication of metal-rubberfriction contacts likely to occur in anti-rim-roll-off systems fortyres. Some of these systems are based on the presence, between the tyreand the rim, of a steel boss on which the tyre comes to rest in theevent of a puncture, which on the one hand allows rolling off from therim to be avoided and on the other allows driving to continue withoutthe need to change the wheel at once. The vehicle can thus travel afurther distance of up to 200 km, at a maximum speed of 80 km.h⁻¹.

However, such a system is effective only insofar as the contact betweenthe metal boss and the rubber tyre is adequately lubricated, in order toavoid a too rapid destruction of the tyre by friction and heating oncontact with the boss. The grease according to the invention representsa particularly suitable solution to this problem: compatible with therubbers of which tyres are made, it allows an effective lubrication tobe ensured between the tyre and the boss. It is thus sufficient to placea layer of grease on the outer surface of the anti-rim-roll-off bossand/or on the inner surface of the tyre. Since the grease according tothe invention can have a high dropping point, it can remain solid at themaximum temperatures (of the order of 160° C.) that are likely to bereached inside a tyre during normal operation (that is to say notpunctured). Thus, the layer of grease does not liquefy, and remains inplace without degrading throughout the life of the tyre until the timeof any puncture (from which moment on it starts to perform its functionof lubricant).

Thus, the invention also provides a vehicle wheel comprising a tyre anda rim, and between them a metal boss on which the tyre comes to rest inthe event of a puncture, and also containing on the outer surface of theboss and/or on the inner surface of the tyre a grease according to theinvention.

The following examples, which are not limitative in character, areintended solely to illustrate the implementation of the invention andthe advantages of the latter.

EXAMPLES

Preparation of Greases According to the Invention:

Grease 1 (Thickened with Lithium Soap):

326 g of sorbitan monooleate and 326 g of polyethoxylated sorbitanmonooleate (POE20) are introduced into a mixer. 180 g of 12-hydostearicacid are then added, and the mixture is heated until the acid melts,then 144 g of a 10% solution of lithium hydroxide are added. The mixtureis then heated to 195° C., then cooled to 80° C. by adding 652 g of themixture of sorbitan/polyethoxylated sorbitan 50/50 by weight. The greaseis then ground in a Fryma colloid mill.

Grease 2 (Thickened with Organic Clay):

582 g of sorbitan monooleate and 582 g of polyethoxylated sorbitanmonooleate are introduced into a mixer. 270 g of organomodifiedbentonite are added, which is dispersed for 1 hour, at ambienttemperature, in the mixture of the two esters of sorbitan. 30 g ofpropylene carbonate are then added, and the mixture is raised to atemperature of 45° C., mixing with a controlled shearing rate. Theproduct is mixed at this temperature until it thickens, after which itis ground in a Fryma colloid mill.

Grease 3 (Thickened with Silica)

652 g of sorbitan monooleate and 652 g of polyethoxylated sorbitanmonooleate are introduced into a mixer. The mixture is raised to 65° C.,and 15 g of Tween 65 (silica-stabilizing agent) are added, then, after15 minutes, 180 g of a silica of a composition suited to polar media.The whole is mixed at 65° C. until it thickens.

Properties and Performances of the Greases According to the Invention:

Table 1 below lists the characteristic properties of the above greases 1to 3.

TABLE 1 Grease 1 Grease 2 Grease 3 Grease 4 PO (in 1/10^(th) mm) 320 288286 303 P6O (in 1/10^(th) mm) 332 302 310 314 P100,000 − P60 26 42 46 /(in 1/10^(th) mm) Dropping point 195° C. >300° C. >300° C. 195° C.

Where:

PO=cone penetrability of the grease, unworked (standard ASTM-D 217)

P60=cone penetrability of the grease, worked, 60 hits (standard ASTM-D217)

P100,000=cone penetrability of the grease, worked, 100,000 hits(standard ASTM-D 217)

The difference P100,000−P60 characterizes the mechanical stability ofthe grease. The dropping point is measured in accordance with standardIP-396.

Grease 4 is a commercial grease mentioned here as a reference. It isconstituted by a polyglycol base thickened with lithium soap, and iscurrently marketed among other things for the lubrication of contactsinvolving elastomers.

The above results show that greases 1 to 3 according to the inventionpresent a good mechanical stability and a high dropping point, withvalues analogous to those of the reference grease.

The performances in terms of lubricating power of greases 1 to 4 weredetermined by tests carried out using a Cameron Plint TE77 reciprocatingtribometer, the operating principle of which is for example described inthe publication “Test Procedure for Rapid Assessment of FrictionalProperties of Engine Oils at Elevated Temperature”, A. G. Plint, M. A.Plint, Tribology International, Vol. 17. No. 4, August 1984, pp.209–213.

These tests were carried out with testpieces made of materialsrepresentative of the metal/elastomer lubrication application, at asliding velocity and initial pressure that were constant andrepresentative of the service, and for various temperature conditionsand periods.

Table II below expresses, in terms of average values of frictioncoefficients (μ) for long-duration tests at 80° C., the relativebehaviour of the 4 greases chosen as examples.

The lower the friction coefficient, the greater the lubricating power ofthe grease.

TABLE II Grease 1 Grease 2 Grease 3 Grease 4 μ_(maximum) (start of test)0.127 0.158 0.147 0.142 μ_(average) between 4^(th) and 0.038 0.031 0.0350.054 14^(th) minutes of test μ_(average) beyond 14 0.035 0.027 0.0270.050 minutes of test

These results illustrate the excellent performances of the greases thatare the subject of the present invention. Greases 1 to 3 have anexcellent lubricating power, which at the end of a few minutes of thetest stabilizes at a level clearly better than that of the referencegrease (grease 4).

1. Lubricating grease, characterized in that it contains: from 70 to 95%by weight of a mixture comprising at least a first ester of fatty acidand of sorbitan and at least a second ester of fatty acid and ofpolyoxyalkylated sorbitan, these esters being present in quantities byweight, relative to the weight of the said mixture, of 10 to 90% and 90to 10%, respectively; and from 5 to 30% by weight of at least onethickening agent.
 2. Grease according to claim 1, characterized in thatthe mixture of the two types of esters contains from 30 to 70% byweight, preferably from 40 to 60% by weight, of the said first ester,and from 70 to 30% by weight, preferably from 60 to 40% by weight, ofthe said second ester.
 3. Grease according to claim 1, characterized inthat the quantities by weight in the mixture of each of the two types ofesters are more or less equal.
 4. Grease according to claim 1,characterized in that said first and second esters are chosen in such away that the mixture of the two types of esters is liquid at 25° C. andatmospheric pressure.
 5. Grease according to claim 1, characterized inthat the fatty acids featuring in the composition of said first andsecond esters contain from 6 to 18 carbon atoms.
 6. Grease according toclaim 1, characterized in that the fatty acids featuring in thecomposition of the said first and second esters are chosen from lauric,palmitic, stearic, oleic acids.
 7. Grease according to claim 1,characterized in that the polyoxyalkyl groups present in the said secondester contain at least one polyethoxy group.
 8. Grease according toclaim 1, characterized in that the said first ester comprises sorbitanmonooleate.
 9. Grease according to claim 1, characterized in that thesaid second ester comprises polyethoxylated sorbitan monooleate. 10.Grease according to claim 1, characterized in that the thickening agentcontains at least a clay grafted by organic groups.
 11. Grease accordingto claim 1, characterized in that the said clay is bentonite.
 12. Greaseaccording to claim 1, characterized in that the thickening agentcontains 70 to 100% by weight of silica.
 13. Grease according to claim12, further comprising from 0.5 to 2 % by weight of an adjuvant, thefunction of which is to increase the thickening power of the silica. 14.Grease according to claim 13, characterized in that the adjuvant ischosen from water, glycerol, ethylene glycerol, glycol, ethylene glycol.15. Grease according to claim 1, characterized in that the thickeningagent content is between 10 and 20% by weight.
 16. Grease according toclaim 1, further comprising 0.25 to 1.5% by weight of an antioxidantadditive.
 17. Grease according to claim 1, characterized in that itscone penetrability is between 265 and 340 1/10^(th) mm.
 18. Preparationprocess for a grease according to claim 1, characterized in that itcomprises a first stage of mixing the said first and second esters, anda second, thickening, stage comprising the incorporation of thethickening agent and the mixture in conditions suitable for bringingabout thickening.
 19. Preparation process according to claim 18, inwhich the thickening agent is based on bentonite grafted by organicgroups, characterized in that the thickening stage is carried out in thefollowing manner: dispersion of the grafted bentonite in the liquidmixture of the two esters, at a temperature of 30 to 60° C. andaccompanied by stirring, heating to a temperature of 30 to 60° C., and,at this temperature, addition of a swelling agent in a quantity ofbetween 5 and 20% by weight relative to the weight of bentonite,stirring at a temperate of 30 to 60° C. at a high shear rate, for 1 to 4hours, incorporation of any additives, final mixing by grinding in acolloid mill.
 20. Preparation process according to claim 18, in whichthe thickening agent is based on silica, characterized in that thethickening stage is carried out in the following manner; dispersion ofthe silica in the liquid mixture of the two esters, at a temperature of30 to 80° C. and accompanied by stirring; incorporation of any adjuvantin a quantity of from 0.5 to 2% by weight relative to the weight of thegrease; mixing at a temperature of 30 to 80° C. at a high shear rate,for 1 to 4 hours, incorporation of any additives and final mixing.
 21. Avehicle wheel comprising a tire on a rim, and between the tire and therim a metal boss on which the tire comes to rest in the event of apuncture, and also containing on the outer surface of the boss and/or onthe inner surface of the tire a grease according to claim
 1. 22. Amethod for lubricating, comprising: providing a grease comprising from70 to 95% by weight of a mixture comprising at least a first ester offatty acid and of sorbitan and at least a second ester of fatty acid andof polyoxyalkylated sorbitan, these esters being present in quantitiesby weight, relative to the weight of the said mixture, of 10 to 90% and90 to 10%, respectively; and from 5 to 30% by weight of at least onethickening agent; placing the grease between two materials.
 23. A methodaccording to claim 22, wherein the materials are included in ananti-rim-roll-off system for tires, and wherein the grease is placed inthe anti-rim-roll-off system.
 24. A method according to claim 22,wherein the materials are selected from a group consisting ofelastomers, synthetic or natural, and plastics.